Lecture 4: Routing. CSE 222A: Computer Communication Networks Alex C. Snoeren. Thanks: Amin Vahdat

Transcription

1 Lectre 4: Roting CSE 222A: Compter Commnication Networks Alex C. Snoeren Thanks: Amin Vahdat

2 Lectre 4 Overview Pop qiz Paxon 95 discssion Brief intro to overlay and active networking 2

3 End-to-End Roting Behavior Importance of paper Revitalized field of network measrement Use of statistical techniqes to captre new types of measrements Empirical findings of roting behavior (motivation for ftre work) Varios roting pathologies Roting loops Erroneos Connectivity altered mid-stream Flttering 3

4 Paxson Stdy: Forwarding Loops Forwarding loop Packet retrns to same roter mltiple times May case tracerote to show a loop If loop lasted long enogh So many packets traverse the loopy path Tracerote may reveal false loops Path change that leads to a longer path Casing later probe packets to hit same nodes Heristic soltion Reqire tracerote to retrn same path 3 times

5 Roting Loops Persistent Roting Loops 10 persistent roting loops in D1 50 persistent roting loops in D2 Temporary Roting Loops 2 loops in D1 21 in D2 Location of Roting Loops: All in one AS 5

6 Paxson Stdy: Cases of Loops Transient vs. persistent Transient: roting-protocol convergence Persistent: likely configration problem Challenges Appropriate time bondary between the two? What abot flaky eqipment going p and down? Determining the case of persistent loops? Anecdote on recent stdy of persistent loops Provider has static rote for cstomer prefix Cstomer has defalt rote to the provider

7 End-to-End Roting Behavior Pathology type Prevalence in 1995 Prevalence in 1996 Long-lived Roting loops ~ 0.14% same Short-lived Roting loops ~ 0.065% same Otage>30s 0.96% 2.2% Total 1.5% 3.4% 7

8 Paxson Stdy: Path Flttering Rapid changes between paths Mltiple paths between a pair of hosts Load balancing policies inside the network Packet-based load balancing Rond-robin or random Mltiple paths for packets in a single flow Flow-based load balancing Hash of some fields in the packet header E.g., IP addresses, port nmbers, etc. To keep packets in a flow on one path

9 Paxson Stdy: Roting Stability Rote prevalence Likelihood of observing a particlar rote Relatively easy to measre with sond sampling Poisson arrivals see time averages (PASTA) Most host pairs have a dominant rote Rote persistence How long a rote endres before a change Mch harder to measre throgh active probes Look for cases of mltiple observations Typical host pair has path persistence of a week

10 Paxson Stdy: Rote Asymmetry Cstomer B Hot-potato roting Other cases Provider B Asymmetric link weights in intradomain roting Cold-potato roting, where AS reqests traffic enter at particlar place mltiple peering points Provider A Early-exit roting Cstomer A Conseqences Lots of asymmetry One-way delay is not necessarily half of the rond-trip time

11 Qestions Why can t we measre the Internet more directly? What can we do abot it? Right division of labor between host and network? For path selection For network monitoring How do we fix these roting problems? In a decentralized, federated network How to incentivize better network management 11

12 Internet Ossification Internet protocols evolve over long timescales Witness the paper we jst read from 1974 IPv6 was proposed almost 15 years ago now! (RFC2460) Yet new ideas appear all the time RFCs are now at SIGCOMM/NSDI/etc. are fll of new protocols/architectres Key challenge: how to deploy new things STAT? In particlar changes to the networking layer Wold naively reqire changing every roter on the Internet! 12

13 Obvios soltion: Overlays Bild the service at a layer above IP Create an overlay of nodes connected by IP Fnctionality implemented at IP end hosts and forwarded on IP provides tnnels between nodes of overlay Exactly the same approach originally sed by IP IP treated the phone network as a series of tnnels Used to introdce a variety of services in the 90s M-Bone, X-Bone, A-Bone, etc., etc. Major downside is performance Each packet needs to be handled by an end host Likely traverses an inefficient rote 13

14 An Alternative Approach Make the roters extensible: Active Networking Provide a mechanism to implement services at an Internet roter Removes the need to rote indirectly Bt still reqires additional end-host-like processing Same idea, two different layers Active nodes are implemented at the (extended) network layer Overlay nodes operate at the application layer Performance/deployment tradeoffs Anybody can deploy an overlay network Bt Active networks cold be mch more efficient 14

15 Active Nodes Execte protocols in restricted environment Limits access to sensitive/shared resorces Primitives for application-defined protocol processing Enforce limits on resorce consmption Active Nodes responsible for network integrity and errors TTL fields decreased as resorces are consmed Capsles with 0 TTL s discarded Code propagation Capsles identify protocol Protocol niqely define code path, Active Node retrieves it MD-5 signatre for safety 15

16 ANTS Java toolkit for writing and execting active protocol code Goals Simltaneos se of mltiple network protocols Deploy mltiple protocols with no central control (orig) Dynamic deployment of new protocols (orig) Migration path from non-active to active world Benefits from a small nmber of active nodes Rather than make hop by hop roting decisions, make active node to active node roting decisions Minimm nmber of nodes necessary for sccess? Small nmber of nodes sccessfl/meltdown nder scale? 16

17 For Next Class Read and review Wetherall 98 Sbmit project ideas by Wednesday We ll post ideas/names to help yo find grops afterwards to me 17